Known in the art is a process for the production of sodium aluminate by crystallizing it from aluminate solutions with a concentration of caustic Na.sub.2 O from 500 to 550 g/l and molecular ratio of caustic Na.sub.2 O to Al.sub.2 O.sub.3 from 20 to 30.
The production of aluminate solutions of the above concentration requires preliminary evaporation of the starting solutions.
The disadvantage of the above-described process consists in an intense corrosion of the equipment, which is due to the use of highly concentrated alkalis, as well as in high power consumption for the evaporation of solutions. Moreover, considerable technical difficulties are encountered in separating sodium aluminate from the mother liquor because of the high viscosity thereof. The end product contains up to 30% of mother liquor, and while washing the latter the crystalline product is partially dissolved.
It is known to produce sodium aluminate by drying an aluminate solution with a molecular ratio of caustic Na.sub.2 O to Al.sub.2 O.sub.3 of 1.1 in the form of a thin film applied to a metal surface at up to 200.degree. C.
In order to prevent aluminum hydroxide precipitate from depositing from the aluminate solution during the drying, up to 5% by weight of organic stabilizers, such as gluconic, gallic, pyrogallic acids or salts thereof are preliminarily added to the aluminate solution (before feeding for drying). The above-mentioned stabilizers are expensive and not readily available products. In addition, the above-described process requires much heat for drying. The end product obtained as a lump layer should be then comminuted for packing, transportation and use. All the above-said makes the process uneconomical.
Known in the art is a process for isolating solids such as calcium chloride or magnesium sulphate from aqueous solutions thereof, the solids being obtained in the form of pellets. The process is carried out in an apparatus having a vertical fluidized bed chamber receiving an ascending flow of heated gas at 200.degree.-370.degree. C. for fluidization of finely dispersed solid particles. The apparatus has a discharge pipe arranged outside the fluidized bed chamber connecting this chamber to a chamber for atomization of the aqueous solution. Finely dispersed particles are recycled between the chambers of the apparatus. An aqueous solution of the solid is sprayed in the atomization chamber on finely dispersed particles heated in the fluidized bed chamber. Thus the solid is released from the solution on the hot surfaces of the solid particles, and water evaporates (that is drying of the solution occurs). The size of the resultant pellets of solid depends on the residence time (number of recycles) of solid particles in the apparatus. Heated gas is admitted to the chamber for atomization of the aqueous solution for transporting solid particles to the fluidized bed chamber. The coarsest particles in the form of pellets which entered the fluidized bed chamber descend to the bottom part of the chamber under gravity and are discharged therefrom. Fine solid particles are again fed to the chamber for atomization of the aqueous solution for repeated treatment.
Drying and pelletizing in the above-described process mainly occur in a flow of finely dispersed particles circulating at transport velocities. To obtain a desired pellet size, it is necessary to provide for repeated circulation of particles through the chamber for atomization of the solution thus increasing power consumption, complicating the equipment and manufacturing process. The need to develop transport velocities for the removal of solid particle from the atomization chamber results in increased dust losses.
Known in the art is a process for the production of a pelletized product, such as monoammonium phosphate or magnesium chloride from solutions or suspensions. In this process, a solution or suspension is atomized by a flow of heated gas at 600.degree. to 1200.degree. C. in the top part of a drying apparatus. The product in the form of pellets dried to between 65 and 95% in the hot gas flow is fed to a fluidized bed for afterdrying to residual moisture content of from 5 to 35% by weight with gas fed through a grate to the bottom part of the apparatus at a temperature below the melting point or decomposition temperature of the end product.
Drying solutions by the above-described method is uneconomical, and this method is only used for products having a low melting point or decomposition temperature. Moreover, the resultant end product has a high residual moisture content which considerably impairs its quality.